build: image: add command to ubinize the kernel image

Newer NAND devices from MikroTik like the hAP ac3
require the kernel to be packed into UBIFS and then
ubinized.

So, since the ubinize-image.sh script can now ubinize
kernel only as well lets add a command for it.

This now allows calling ubinize-kernel in the kernel
packaging at then end.

Signed-off-by: Robert Marko <robimarko@gmail.com>

include/image-commands.mk

@@ -311,6 +311,18 @@ define Build/append-ubi
 	rm $@.tmp
 endef
 
+define Build/ubinize-kernel
+	cp $@ $@.tmp
+	sh $(TOPDIR)/scripts/ubinize-image.sh \
+		--kernel $@.tmp \
+		$@ \
+		-p $(BLOCKSIZE:%k=%KiB) -m $(PAGESIZE) \
+		$(if $(SUBPAGESIZE),-s $(SUBPAGESIZE)) \
+		$(if $(VID_HDR_OFFSET),-O $(VID_HDR_OFFSET)) \
+		$(UBINIZE_OPTS)
+	rm $@.tmp
+endef
+
 define Build/append-uboot
 	dd if=$(UBOOT_PATH) >> $@
 endef